I know with hickory, the white wood is typically used for tension. That said, I've made a bow out of all heartwood and a bow out of all sapwood, but never one with a sapwood back and heartwood belly. In my case, I found both heartwood and sapwood to be very forgiving of mistakes and very tough. Someone with experience please correct me if I'm wrong, but they seemed about equivalent for me. Didnt matter which I used, the wood behaved about the same.